First off, I would try to run out of Prime rib before being faced with a reheat but like anything else, it doesn't always go as planned.

So, that in mind, and with the process of searing the whole and cooking 10 minutes per pound at 225 with a hold at 120 for 4-6 hours to start a fresh rib..........I'll go out on a limb with this suggestion.

Individual portions can be difficult to reheat and still retain the color/temp. However, something that worked fairly well for me was placing the portion on a warm sizzle platter or saute pan, add some Au jus and then cover with leaf lettuce. I've used both the oven and salamander with success. Basically, the leaf lettuce acts as an insulator and allows the meat to come up to a more desirable temp. Doesn't do much for the leaf lettuce but it does help to protect the portion from changing too much.

If it's un-cut, I've done a couple of these at home over the years and use the lowest possible setting of the oven, usually 170 and took my chances. Cooking at home gives you more flexibility with temp zones so I was able to get a close rare with no issues to speak of.

As far as in the restaurant, maybe put the uncut rib in the sham for the last 15-20 minutes of the cook cycle before hold. This applies to a rib of 1/2 or more of the whole remaining and even then it was mainly used for med and above cuts. Every so often we'd get lucky with one or two rare cut but it was never planned. Anything smaller than 1/2 was either shaved for Prime and Cheddar sands or cut for Grilled prime rib sands.

I can only guess the eye's are already rolling but seriously, it doesn't work all that bad.

Been there and done that. I used to wrap the whole rare ribeyes in an oiled wrapping paper then into a slow oven to bring up to a warm temperature. Removed from the bag, turn up the heat and allow the roast to crisp again. Sounds weird but it worked out well.

OTOH.....whenever I have been in a situation where I was served a reheated prime rib......it had a distinct flavor that I will remember forever.

For individual portions I reheat it in the microwave in a deep plate with au jus. It will never be rare again, it's almost always cooked until the pink is almost gone but the meat is tender so I don't mind left over prime rib. I've also seared it in a skillet like a steak and that does a better job of retaining the pink center but it sears like a steak.